let a = '9007199254740991';
let b = '1234567899999999999';
function add(a, b) {
	//取两个数字的最大长度
	let maxLength = Math.max(a.length, b.length);
	//用0去补齐长度
	a = a.padStart(maxLength, 0); //"0009007199254740991"
	b = b.padStart(maxLength, 0); //"1234567899999999999"
	//定义加法过程中需要用到的变量
	let t = 0;
	let f = 0; //"进位"
	let sum = '';
	for (let i = maxLength - 1; i >= 0; i--) {
		t = parseInt(a[i]) + parseInt(b[i]) + f;
		f = Math.floor(t / 10);
		sum = t % 10 + sum;
	}
	if (f !== 0) {
		sum = '' + f + sum;
	}
	return sum;
}
console.log(add(a, b))
